Process for producing low alloy steel for oil well tubing and tubing thereof



March 4, 1958 Filed June 17, 1955 E. M. HERZOG PROCESS FOR PRODUCING LOW ALLOY STEEL FOR OIL WELL TUBING AND TUBING THEREOF 4 Sheets-Sheet 1 Conescr STRUCTURE 2 5470s: 5755; A

.Ameusn 760 C. 31100125,

Ouavcuza 980 C.

TEMPERED 31/0005 AT 650-0.

MAs/v/r/ur/a/v x 600 YIELD Pom/r 8.9000 R5]. ULTIMATE STRENGTH 98000 RS.

I/o Emu/v55 W/rnnv 0000 HOURS, Jonas TEsr CORRECT 5717mm: 4" 7035' STEEL A ANNEALED 750 C. 5HOURS, Ousucmzv 1020 6. TEMPERED 5 HWRS AT 650-"C. YIELD Pamr 02000 RSJ. ULTIMATE STRENGTH 98000 RS. I.

No BULL/RES WITH/N 7800 HOURS MAe/v/F/cA r/0/v X 600 Ac/cuzm Smucrunz 2 a fuaz STEEL A Ouavcmso 970 C.

'TEMPERED 625 C. 6 110005 MA emf/cum X 500 YIELD Pam/r 92500 P.5Zl.

UL 1/ MA rs Smmvem 773.5 00

fn/Lunss Wmw/v 744 #0005 nv VENTOA. EUGENE M Hzezos.

March 4, 1958 E. M. HERZOG 2,825,669

PROCESS FOR PRODUCING LOW ALLOY STEEL FOR OIL WELL TUBING AND TUBING THEREOF Flled June 1'7, 1955 4 Sheets-Sheet 2 1 COARSEA/ED GPA/IV, i715 CARBIDE IYETWORK 0N Boa/v04 R155 2 /8 22/35 STEEL A MAGN/F/CA TI0Iv X 500 OUE/VCHED AT 970%.

TEMPERED 6 HOURS A7 62.5%? YIELD 'PoIIvT 62.000 P..S./.

ULTIMATE STRENGTH 92000 t P. 5. I.

' FAILURE W/TI-I/IV 7Z0 HOURS As SAND CAST 9235M BAR MAG/l/F/CAT/OIV X 500 YIELD POI/V7 46.000 P.5 ULTIMATE STRENGTH 72500 FAILURE W/7H//V 98 HOURS Comss GRAIN STRUCTURE Assn-canes OFC'ARBIDEs, FEE/HIE lA/VE/VTCI? EUGENE M HEPZOG.

March 1953 E. M. HERZOGY 2,825,669

PROCSEISLE g g-1.2L P II KLOBDUCING LOW ALLOY STEEL FOR ING AND TUBING THERE Filed June 17, 1955 4 Sheets-Sheet 4 lacUuk STRUCTURE As 0157' m/ lea/v Mal/L0 QUENCHED AT 950 C. V

I'EMPERED 2 HOURSAZ650-C MAGN/F/CAT/O/V X 500 YIEL'U POI/VT 62.000 P.5./.

ULTIMATE STRENGTH 98.000

R S. I.

FAILURE W/rH/U 190 HOURS Ac/cULAR STRUCTURE, FULL Y,

TEMPERED Bm/rurrm warm/m Comgcr STRUCTURE As Cnsr //v //?0/V Mouw ANNEALED 4001/25 A7 760C.

QUEIVCl-IED A7 950 C.

TEMPERED 2 Hal/RS A7 650 C.

Y/ao POI/II 80.500 P51.

ULTIMATE STRENGTH 96.000

I/o Emu/ass W/rrmv 8000 #00175 MIG/l/F/CATIOIV X 500 'ULLY Somme STRUCTURE IN VENTOR. EUGENE M. HEEZOG.

dwma atmwm United States Patent O PROCESS FOR PRODUCING LOW ALLOY STEEL FOR OIL WELL T UBENG AND TUB- ING THEREOF Eugene Herzog, Nancy, France, assigner tn Soeiete des Acleries de Pompey, Pompey, France, a French body corporate Application June 17, 1955', Serial No. 516,275

Claims priority, application France July 13, 1954 8 Claims. (Cl. 148-2155) This invention pertains to a low alloy steel of novel analysis, and to a novel method of heat treating the same for imparting thereto high strength and a microstructure which is resistant to hydrogen sulfide stress-corrosioncracking, as well as to cast, forged and rolled products made of such steel, heat treated to produce the properties aforesaid.

The steel of the invention is particularly applicable to the production of any kind of steel articles among which steel tubing should be mentioned. The said steel tubing is heat treated in accordance with the novel heat treating processes of the invention, for imparting high strength and high resistance to hydrogen sulfide stress-corrosion cracking.

Steel tubing for the above applications is usually rolled, forged or cast from steels having a high tensile strength in order to withstand the high stresses encountered in columns of tubing built up in oil wells, the depth of which may range from about 2,000 to 14,000 feet.

Under certain corrosive and stress conditions, such tubing is liable to fail suddenly by cracking, for example, when the escaping gases are high in hydrogen sulfide, the proportions of which may range up to 10%, 20%, or 30% H 8 in the well gases. As is known from recent research investigations, quenched and tempered steels for such applications, having a fully or partially tempered martensitic structure, are prone to stress-cracking corro- V being generated by contact between the iron and moist hydrogen sulfide of the well gases, according to the reaction: Fe+H S FeS+2I-I.

Under such conditions hydrogen difiuses into the steel and reduces the tensile elongation as well as the reduction in area, with progressive embrittlement of the steel the longer it is subjected to such conditions. This progressive embrittlement is accelerated by straining and stressing of the steel in the presence of the corrosive action of moist hydrogen sulfide. In the steel as thus embrittled, the capacity for deformation or plastic flow is substantially nil. The hydrogen concentration in the steel is proportional to the square root of the gaseous pressure of the hydrogen sulfide or atomic hydrogen present.

High tensile stresses thus produced in such hydrogen saturated steel, open up micro-cracks along the grain boundaries at which carbides are situated, and also at internal porosities or segregations in the steel, which microcracks thereupon spread out into large intergranular cracks or fissures extending across the grains with resulting failure under stress.

2,825,669 Patented Mar. 4, 1958 In a previous copending patent application, it has already been proposed to use low alloy steels containing chromium and aluminum with addition of several other elements such a molybdenum, manganese, titanium, vanadium, etc. and to subject these steels to a heat treatment consisting on the one hand of an austenite-forming step followed by an air cooling at a speed at least equal to that corresponding to a cooling at room temperature, and on the other hand of an annealing or tempering operation lasting a couple of hours. Such a heat treatment, when applied to steels of this kind, permits of obtaining a microstructure having the properties required for avoiding the intergranular stress-corrosion-cracking of moist gases such as hydrogen sulfide.

A primary object of this present invention is to still further improve the steel and steel articles thus obtained.

Another object of this invention is to provide a steel article and particularly a heat treated tubing for oil wells and refinery which is resistant to wet hydrogen sulfide gases and aqueous solutions thereof under gaseous pressures of up to 1,400 p. s. i. (partial H 5 pressure being of 500 p. s. i. for instance) in the temperature range of 0 to C. and under tensile stresses of up to 75,000 p. s. i. It is of course indispensable to hear these considerations in mind, as even a very local failure can mean a total collapse of a tubing.

In order to meet these conditions, the steel of this invention must be heat treated to have an ultimate strength of at least 95,000 p. s. i., a 0.2% offset yield strength of at least 75,000 p. s. i., a tensile elongation of at least 16% and a reduction in area of at least 70%.

The steel of this invention comprises, as stated, a low alloy steel which is heat treatable to produce a specific microstructure, as set forth below, said steel containing as essential constituents, in addition to an extremely close tolerance of carbon, small amounts of chromium, manganese, aluminium, silicon and preferably also small amounts of molybdenum, vanadium and titanium. I

I have observed that in certain cases the crystalline structure of the pieces or articles which were obtained by rolling, moulding or forging showed a great brittleness due to the frequent presence of the constituent known under the name of bainite in a partially annealed state; Generally speaking, the difliculty was due principally to big aggregates of carbides disposed around the grains or the crystals of the mass. Practice has also shown that the unequal distribution of carbides in the sections of the different articles was subsisting even after very long heat treatments attaining several hours and executed in the region of the austenite temperatures which attain 950 to 1050 C.

One of the principal objects of this invention is to bring a remedy to this situation.

The method according to this invention is chiefly characterized in that it consists to employ a steel containing chromium and aluminium with or without the addition of other elements and to produce a diffusion or dispersion. in the mass of the carbides and of their aggregates by heating the articles at a temperature lying beneath that of the transformation of on iron into 7 iron.

According to another featurenof this invention, the steel'article is used either in the state as it stands after this dispersion or diffusion treatment, or upon a subsequent normalizing or austenitizing heat treatment, fol lowed by an annealing action, for instance as described hereabove. V

In fact, I got aware of the fact that a heating at about 730 to 800 C. for instance, was able to realize a diffusion or a dispersion of practically all the carbides and of all .their aggregates in the mass of the steel article. It has alsobeen observed that after it had'been'subjected tosuch a treatment, the steelhad aqmte uniform and horriog'eneous structure capable to respond to a suitable heat treatment, in much better condition and with a greater success than if it were treated immediately after the rolling, the

casting,or the;forging i ,I/have, also observed that suchaiprelinnnaryrheatingi which couldobetermedflilfusion or dispersion'heatinghad- .to;last s'everalhoprs, the number of honrszdependin'griof:

courseupon the nature of. the steel; andthe: size ofthe pieces and upon;the mechanical properties'reqnired: of. the finalarticle... Should this treatrnentinotbe applied to the steel article,.the danger would. exist ofobtaining a. partially tempered bainite-havinga dendritic or acicular form .and having an inferior resistance. to .hydr'og'en sul-n phide gas stress-corrosion-cracking.M r 011 1116 contrary,.thispreliminary dispersion'heating of the carbide' aggregates. results in a ,structurecontainingfi ferrite and very finely 'divided carbidesl The above-mentioned'prronheat treatmentconsisting of an austenitic or normalizing heating and-quenchingfol- Iowed'by'an annealing,- canbe very successfully applied to such preliminary treated steel artices, giving themthenecessary mechanical features and permitting them to withstand the. corrosive action or: hydrogen sulphide gasl andlalso' the high pressures in the oil wells, etc.

It can be observed that the tempering which-is usedafter the austenitizing step gives the steela structural stabbility with regard to the corrosive action of the gases.

My investigations have also established that the upper limits for carbon, manganese and chromium; in the steelof my invention, must be carefully limited,- because-these eleme'nts tend to enhance hydrogen-embrittlemerit'and' hydrogen sulfide intergranular stressecorrosion-cracking.

Although these-elements are required forimpar ting therequisite tensile properties to the steel inthe heat' treated condition, these properties must be obtained-by the lowest amounts of these elements that can -be employed in the steel with regard to this; special stress-corrosion resist-- ance'.

' 1 Nickel is deleterious to the steel of-my invention andispreferably absent and, when present-shall be under 0.2%. The steel of the invention should,- for the same reason, be low in manganese, under 1.3 Likewise the chromium present in the steel could be-urider 3%. The

presence of molybdenum is useful over the range of about 0.3-0.5 maximum, as higher amounts-increase the duration'ofthe heat treatmentrequired forimparting the: requisite micros'tru'cture to the" steel, as set forth below; Aluminiumisrequired as anelement promoting'and ac-' celerating migration and dispersion of carbides into the ferrite grainsgfor imparting stress-corrosion resistance and ferrite strengthening. Vanadium and molybdenum are carbon binders and'ferrite strengthenersandare prefe'rential in their action on these aspects as compared to" chromium; The tolerance on carbon is extremely close;

because if the carbon is too low, the steel cannot be heat treated to the requisite strength level, and, on' the otherhand, if-the carbon is too high, the resistance to stresscorrosion-cracking is impaired.

Therbroad' range of analysis of steel according to the invention is;

Very'good resultsthave been realizedwithsteels having Percent by weight;

compositions falling in the following narrower range of analysis: v

Percent by weight The balance of the steel is iron except for residual elements within usual commercial tolerances. For example, copper should be less or equal to 0.40%, nickel less than 0.2%, sulfur not over 0.03%, phosphorus not over 0.04% maximum, nitrogen not over, 0.02%, oxygen not over 0.01%. n

Within the aforesaid broad range of analysis of steel according to the invention, two types or grades having the properties set forth below are obtained within the following ranges of analysis, respectively, these gradesbeing designated herein as steels A- and B.

Steel-'14 Percent bywei nt Preferred 0.12 Preferred 0.50 Preferred 2.20 Preferred 0.30 Preferred 0.40 Preferred 0.35; Preferred 0.10

Preferred .010 Preferred .025

Preferred .005' Preferred .020

iron

Percent byiwe ig ht; V

20 Preferred 0.1 .60-1 20 Preferred '1. 60-1. 20 Preferred 1. 10-0. 50 Preferred 0. -0 60 0..

Aluminium; With or without- Molybdenum.

Vanadium..

Balance;. .Q.

P ef rm Preferred 20 Preferred 10 7 iron 7 Additions of one or more of vanadium, boron, cerium; titanium to the steel of the invention increasethe tens'ilei strength thereof;

Seamless tubing is made of the steeloflhelinvention'. by hot piercing the steel at a temperature offlaboutj 1150-1200 C., and thereafter drawing, as conventionally produced. 7 Q

The tubing as thus produced must be thereaftercar'e-i fully heat treated as setforth below, for reasons as" follows: 7 I i As above mentioned my investigations have also shown that a uniform microstriicture is essential in orderto part high resistance to stress-corrosion-cracking over'long'. periods of time. l'Ihemicrostructur'e must be such that} the hydrogen shall difiu'se withsubstantiallyequal facility into all portions :ofthe steel. If the microstructu're'is'" such'that such uniform diffusion of atomic hydrogen does conventional procedures, can neverthelessfail' by botli ofF the above described cracking processes. I have found that the required microstructure for eliminating this danger must embody the following characteristics: (1) a uniform distribution of the carbides in small particle size of not more than 5 microns uniformly dispersed throughout the ferrite grains with a minimum of carbides situated at the grain boundaries; (2) the heat treated steel should contain preferably none and certainly not more than 5% of f ee ferrite; and (3) the heat treated steel should also preferably be entirely free of acicular bainitic structures and must certainly contain not more than of such structures. The heat treated steel should also contain less than 0.01% of carbon dissolved in the ferrite.

The undesired microstructures above mentioned are normally present to the extent of about 70 to 90% in forged and cast parts after conventional quenching and tempering treatments. In hot rolled products such as tubes, bars, plates, etc., they are invariably present to the extent of about l050%. Fractures or cracking of such structures occur at between 300 to 3500 hours of service life as compared to a useful life of 6,000 to 8,000 hours without failure if the steel of the present invention, properly heat treated, is employed.

The desired microstructure of the steel of my invention is produced by the following sequence of heat treatments: First the steel is given a preliminary low temperature anneal by soaking at about 740780 C., in order to assure uniform dispersion of the carbide aggregates and their diffusion throughout the ferrite grains. If the steel is annealed below this temperature range the dilfusion is too slow, and if the annealing is above this temperature range, the carbides go into solution and form intermediate austenitic constituents which are higher in carbon than the surrounding pre-existing ferrite grains, so that the carbide aggregates are not uniformly dispersed. There accordingly results localized high carbon and low carbon areas which are retained even after quenching. According to eir dimensions, the cast, forged or rolled parts made of the steel of the invention are held at an average of about 750 C. for about 8 hours per inch of thickness of the material. For rolled products annealing requires at least three hours for each /2 of thickness of the material.

The second step in the heat treatment consists in drastic quenching, as in water, after heating to about 970- 1080 C. The soaking time at this temperature is about 1 hour maximum per inch of thickness of the material at 970 C., and about minutes per inch of thickness at 1080" C. Quenching from about ll00 C. results in grain coarsening and overheating so that on subsequent tempering, as noted below, there results an acicular and coarse grain microstructure, which is undesired for reasons above discussed.

After the quenching treatment the steel is subjected to a very careful tempering heat treatment in order to assure complete decomposition of the hardened constituents resulting from the quenching operation, such as martensite, bainite, etc., and in order to assure the ejection of the carbon from the ferrite and to produce uniform dispersion of the carbides throughout the ferrite grains with a minimum of carbide segregation at the grain boundaries. To assure this, the tempering should be effected in the temperature range of about 625-670 C. If the tempering is carried out, for example, at 650 C., the tempering treatment should be continued for about 3 hours for each /2 of thickness of rolled tubing. If this treatment is efiected at 625 C., then the tempering should be continued for about 8 hours for each /2" of thickness of the tubing.

Checking for correctness of the heat treatment is performed by first tensile testing the heat treated material to determine if it meets the required tensile properties as above noted.

Hydrogen embrittlement is tested by electrolizing the tensile specimen in 5% sulfuric acid at room temperature, the specimen being the cathode of the cell and charged by hydrogen generated by a current density of 10 milliamperes per square centimeter for a cross section of the steel of 12.6 square millimeters.

However my investigations have shown that this test is not reliable for resistance to H 8 stress corrosion testing. Steel known on the market under the standard designation N being normalized and tempered and stressed up to its 0.2% offset yield strength in moist H 8, fails within 1 to 2 hours.

The steel according to my invention withstands 40 hours without rupture.

The same specimens, stressed to their yield strength in sulfuric acid do not fail within 48 hours. After cathodic charging by hydrogen (l0 ma./cm. both steels were broken within 45 to 120 run. This means that electrolytic testing in acids cannot give a true picture of steel behaviour in H 8.

Resistance to hydrogen sulfide stress corrosion is tested by the so-called "Jones device at atmospheric or higher pressures.

In the accompanying drawings:

Figure l is a photornicrograph, at 600 magnification, of 2 /8 tubing made of Steel A according to the invention, which has been given the correct heat treatment to produce the correct microstructure above described, the tubing having been annealed at 750 C. for three hours, thereupon heated to 980 C. and water quenched, and thereupon tempered for three hours at 650 C. The steel as thus heat treated had a 0.2% offset yield strength of 89,000 p. s. i., and, ultimate strength of 98,000 p. s. i., and, when tested by the Jones test, showed no failures within 6,000 hours.

Figure 2 is a photomicrograph, at 600 magnification, of 4 tubing having the analysis of Steel A, heat treated to have the correct microstructure as produced by annealing at 750 C. for three hours, thereupon heating to 1020 C. and water quenching, followed by tempering at 650 C. for three hours. As thus heat treated, this steel had a 0.2% offset yield strength of 82,000 p. s. i., an ultimate strength of 98,000 p. s. i., and, when tested by the Jones test, showed no failures within 1,800 hours.

In contrast to the above, Figure 3 is a photomicrograph, at 500 magnification, of 2% tubing having the analysis of Steel A, but showing the incorrect or acicular microstructure resulting from simply quenching and tempering in accordance with conventional practice. In this case the tubing was simply heated to 970 C. and water quenched and thereupon tempered at 625 C. for six hours. The 0.2% offset yield strength was 97,500 p. s. i., the ultimate strength 113,500 p. s. i. and the steel failed after 744 hours of testing by the Jones test. The undesired acicular microstructure is evident from the photomicrograph.

Figure 4 shows another specimen of this same tubing as heat treated to give a coarsened grain, fine carbide network on the grain boundaries as produced by simply quenching from 970 C. and thereafter tempering at 625 C for six hours. As thus heat treated, the steel of the tubing had a 0.2% ofiset yield strength of 82,000 p. s. i., an ultimate strength of 97,000 p. s. i. and failed within 720 hours when subjected to the Jones test. The undesired acicular microstructure is evident from the photomicrograph.

Figure 5 shows a steel of the analysis of Steel A, at 500 magnification, as sand cast into a mm. diameter bar.

As thus sand cast the steel had a 0.2% offset yield strength of 46,000 p. s. i., an ultimate strength of 77,500 p. s. i. and failed within 98 hours when subjected to the Jones test.

Figure 6 shows the steel of analysis A as sand cast into a 95 mm. bar and thereafter annealed for four hours at 780 C. As thus sand cast and annealed, the steel had an 0.2% ofiset yield strength of 43,000 p. s. i., an ultiafter. 8,000 hours when tested by the Jones test.

Figure 7 shows, at 500 magnification, steel'of analysis 'A as sand'cast into an iron mold. The steel as thus cast hada 0.2% offset yield strength of 30,000 p. s. i., an ultimate strength of 67,000 p. s. i. and failed within 112 hours'when tested by the Jones test with coarse ferrite, aggregates of carbide,

'Figure 8 shows, at 500 magnification, the steel of analysisA as cast in an iron mold and thereafter annealed for four-hours at780 C. The 0.2% offset yield strength was 47,500 p. s. i., the ultimate strength 70,500 p. s. i.

and the steel showed no failures after 8,000 hours of testing. The microstructure is ferrite with dispersed carbides;

Figure 9 shows, at 500 magnification, the steel of analysis A as cast in an iron mold, thereupon heated to 950 C.

and water quenched, followed by tempering for two hours at 650 C. 'The 0.2% offset yield strength was 82,000 p. s. -i., the ultimate strength 98,000 p. s. i. and the steel failed within 190 hours when subjected to the Jones test. The undesired acicular fully tempered bainitic microstructure is clearly evident.

Figure 10 shows a steel of analysis A as cast in an iron 'mold, thereafter annealed for four hours at 780 C.,

thereupon heated to 950 C. and water quenched, and thereupon tempered for two hours at 650 C. This steel again shows the correct and desired sorbitic microstrueture. The 0.2% offset yield strength was 80,500 p. s. i., the ultimate strength of 96,000 p. s. i. and the steel showed 'no failures after 8,000 hours of testing by the Jones test.

What I claim is:

1. In a process of treating'fabricated low alloy steel articles having as essential constituents in addition to iron, 0.08%0.20% of carbon, 0.15 %1.20% of aluminum, 0.6%5.0% of chromium, the step of subjecting the said fabricated article to a soaking intended .to produce adispersion or a diffusion in the massof said steel of the carbides contained therein together with their aggregates and boundaries, said soaking being executed at a temperature lying somewhat below the temperature of transformation of ec-iron into 'y-iI'OB, the said soaking temperature for the above contemplated-steel range being situated at about 730 to 800 C. I

2. In a process according to claim 1 the featureresiding in the incorporation inthe steel of 03-120 of manganese, 0.100.60 of silicon, up to 0.50% of molybdenum, of up to 1.00% of vanadium and of up to 1.00% of titanium.

3. In a process of treating fabricated low alloy steelv and boundaries, said soaking being executed at a tern perature lying somewhat below the temperature of transformation of oc-iron into 'y-iron, the said soaking temperature for the above contemplated steel range beingsituated at about 730 to 800 C. and still the further step consisting in subjecting said article aftersaid diffusion producing soaking to' a further heat treatment intended to conferto' said article the required mechanical features, said treatmentcomprising the formation .of austenite by a heating at about 9501080 C. followed by a quenching and an annealing at about 600700 C.

4. In a process according to claim 3 the feature residing in the incorporation in the steel of 0.31.20 of manganese, 0.10-0.60 of silicon up to 0.50% of molybdenum, of up to 1.00% of vanadium and of up to 1.00% of titanium. 7

5. Fabricated low alloy steel articles containing in addition to iron, 0.08%0.20% of carbon, 0.15%1.20% of aluminum and 0.6%5.0% of chromium, said fabricated low alloy steel articles being subjected to a soaking at aternperature lying somewhat below the temperature of transformation of oc-ilOIl into 'y-iron, the soaking temperature for the above contemplated steel range being situated at about 730800 'C.,'the said soaking being intended to produce a dispersion or diffusion in the massof said steel of the carbides contained therein together with their aggregates and boundaries.

6.Fabricated low alloy steel articles containing in addition to iron, 0.08%-0.20% of carbon, 0.l5%1-.20% of aluminum and 0'.6%5.0% of chromium, said fabricated low alloy steel articles being subjected to a soaking at a temperature lying somewhat below the temperature of transformation of a-iron into 'y-iron, the soaking temperature for the above contemplated steel range being situated at about 730-800 C., the said soaking being intended to produce a dispersion or diffusion in the mass of said steel of the carbides contained therein together with their aggregates and boundaries, the .said soaking being followed by a further treatment consisting in subjecting said article after said djfiusion'producing soaking to a heat treatment intended to confer to said article the re? quired mechanical features, said treatment comprising the. formation of 'austenite by ;a heating at about 950- 1080" C. followed by a quenching and an annealing'at about 600-700 C. 1 1.

7. Fabricated low alloy steel articles according to claim 5 wherein the steel further contains 0.3 1.20% of .manganese, 0.10-0.60% of silicon, up to 0.50%; of molyb: denum, up to 1.00% of vanadium and up to 1.00% of titanium. V i

8. Fabricated low alloy steel articles according to claim 6 wherein the steel further contains 0.3-1.20% of manganese, 0.100.60% of silicon, up to 0.50% of molybdenum, up to' 1.00% vanadium and up' to 1.00% of titanium. r

References Cited in the file of this patent OTHER REFERENCES Steel Processing, The Microstructure of Low Carbon Steel, November 1948, pages 605-607. 

1. IN A PROCESS OF TREATING FABRICATED LOW ALLOY STEEL 0.08%-0.20% OF ALUMINUM, 0.6%-5.0% OF CHROMIUM, THE STEP OF SUBJECTING THE SAID FABRICATED ARTICLE TO A SOAKING INTENDED TO PRODUCT A DISPERSION OR A DIFFUSION IN THE MASS OF SAID STEEL OF THE CARBIDES CONTAINED THEREIN TOGETHER WITH THEIR AGGREGATES AND BOUNDARIES, SAID SOAKING BEING EXECUTED ATG A TEMPERATURE LYING SOMEWHAT BELOW THE TEMPERATURE OF TRANSFORMATION OF A-IRON INTO R-IRON, THE SAID SOAKING TEMPERATURE FOR THE ABOVE CONTEMPLATED STEEL RANGE BEING SITUATED AT ABOUT 730 TO 800*C. 